A magnetic memory (hereinafter also referred to as a magnetoresistive random access memory (MRAM)) is a nonvolatile memory capable of high-speed operation. Therefore, MRAMs are expected to serve as novel nonvolatile work memories, and are being developed by numerous organizations. An MRAM includes a magnetic tunnel junction (MTJ) element as a storage element. This MTJ element includes a first magnetic layer having a fixed magnetization direction (this magnetic layer is also referred to as the reference layer), a second magnetic layer having a variable magnetization direction (this magnetic layer is also referred to as the storage layer), and a nonmagnetic insulating layer disposed between the first magnetic layer and the second magnetic layer.
Writing into the MTJ element is performed by switching the magnetization direction of the storage layer. One of the known techniques for such writing is spin transfer torque magnetization switching (hereinafter also referred to as spin transfer torque (STT)). By this technique, current is applied from the storage layer to the reference layer, or from the reference layer to the storage layer, via the nonmagnetic insulating layer. In this manner, the magnetization direction of the storage layer is switched.
In writing by this STT, current is applied to the nonmagnetic insulating layer. As a result, the nonmagnetic insulating layer might be broken at a time of writing. Since current is also applied to the nonmagnetic insulating layer at a time of data reading, read disturb might occur, as the magnetization direction of the storage layer is switched by STT at a time of data reading.
Another one of the known techniques for writing is a technique using a spin Hall effect or a spin-orbit interaction. A spin-orbit interaction is a phenomenon in which current is applied to a nonmagnetic layer so that electrons having spin angular momenta (hereinafter also referred to simply as the spin) of the opposite orientations from each other are scattered in the opposite directions, and a spin current Is is generated. At this point, the spin s, the spin current Is, and the electron current Ie (of the opposite direction from the current) satisfy the relationship:Is∝s×IeThat is, the spin current Is is proportional to the cross product of the spin s and the electron current Ie. As an MTJ element is stacked on the nonmagnetic layer, spin torque (spin orbit torque (SOT)) is applied to the storage layer of the MTJ element by virtue of the spin current generated in the nonmagnetic layer, and the magnetization direction of the storage layer can be switched. An MRAM that performs writing by using this principle is called an SOT-MRAM.
An SOT cell that is a memory cell used in an SOT-MRAM has a three-terminal element in its fundamental configuration. Since a read current path and a write current path are different, two or three transistors are provided for one SOT cell. Therefore, the cell area becomes larger.
Further, there is a reported technique by which MTJ elements and one transistor are stacked on one nonmagnetic layer, for example. To perform writing on a certain cell according to this technique, current needs to be applied in the in-plane direction to the nonmagnetic layer while spin torque is applied to the MTJ elements by applying current in a direction perpendicular to the stacking direction of the MTJ elements. Because of this, controlling the write current value becomes more difficult. Furthermore, since MTJ elements are integrated on one nonmagnetic layer, the nonmagnetic layer inevitably becomes longer, and the possibility of electromigration might become higher.